1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine.
2. Discussion of the Background
An exhaust emission control device including a catalytic converter configured to purify exhaust has been traditionally arranged in the exhaust system of an internal combustion engine. The catalytic converter includes an exhaust purification catalyst configured to reduce carbon monoxide (hereinafter, referred to as “CO”), non-methane organic gases (hereinafter, referred to as “NMOG”), and nitrogen oxides (hereinafter, referred to as “NOx”) in the exhaust.
A common catalytic converter includes a support and an exhaust purification catalyst supported on the support. More specifically, a common catalytic converter includes a support composed of a ceramic material or a metal, the support having a honeycomb structure that includes a plurality of cells therein; and an exhaust purification catalyst containing noble metals, such as platinum (hereinafter, referred to as “Pt”), palladium (hereinafter, referred to as “Pd”), and rhodium (hereinafter, referred to as “Rh”), the exhaust purification catalyst being supported on the support (see, for example, Japanese Unexamined Patent Application Publication No. 2007-278100).
The noble metals, such as Pt, Pd, and Rh, are known to be very expensive. So, the amount thereof used is required to be minimized from the viewpoint of reducing the cost of the exhaust purification catalyst. FIG. 12 is a graph illustrating annual changes in the prices of Pt, Pd, and Rh. As illustrated in FIG. 12, among these noble metals, Rh is the highest average price, and the price fluctuation range of Rh is large. So, a reduction in the amount of Rh used is strongly required.
The inventors have reported a technique in which the amount of Rh used is reduced without reducing exhaust purification efficiency (see, for example, WO2010/61804). In the technique described in WO2010/61804, a catalytic converter unit configured to purify exhaust includes two converters: an upstream catalytic converter and a downstream catalytic converter. The amount of Rh in the downstream catalytic converter is larger than that in the upstream catalytic converter. The degree of poisoning of Rh by phosphorus and sulfur in exhaust is increased when Rh is arranged at a more upstream portion, thus causing a reduction in the purification performance of Rh. In this technique, the small amount of Rh used in the upstream catalytic converter suppresses the poisoning of Rh, thereby reducing the amount of Rh used.
Furthermore, in the technique described in WO2010/61804, the exhaust purification catalyst in the upstream catalytic converter has a three-layer structure. Rh is arranged in layers other than the outermost layer. The degree of poisoning of Rh is increased with decreasing distance from the surface. In this technique, Rh is arranged in the layers other than the outermost layer, thereby suppressing the poisoning of Rh and reducing the amount of Rh used.
Moreover, in the technique described in WO2010/61804, the amount of an oxygen storage component (hereinafter, referred to as an “OSC”) in the upstream catalytic converter is larger than that in the downstream catalytic converter. The OSC has the capability of occluding oxygen in an oxidizing atmosphere and releasing oxygen in a reducing atmosphere. In this technique, a larger amount of the OSC is arranged in the upstream catalytic converter. So, the air-fuel ratio of exhaust passing through the downstream catalytic converter is stably controlled in the vicinity of the stoichiometric ratio, thereby improving the exhaust purification efficiency of the downstream catalytic converter.